The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method. For example the United Kingdom Patent Specification 1,241,661 discloses a method for the production of a planographic printing plate consisting of a sheet material comprising an outer hydrophilic colloid layer on the surface of which there is concentrated a silver image stemming from an underlying exposed silver halide emulsion layer by the silver complex diffusion transfer reversal process. The silver image formed on the surface is suitable for printing images in a lithographic printing process using a dampening liquid.
Commercial lithographic printing plate precursors of the latter type typically contain on a support in the order given a base layer serving as an anti-halation layer, a silver halide emulsion layer and a surface layer containing physical development nuclei in which the silver image is formed.
Such lithographic printing plate precursors, also called imaging elements may be camera exposed or they can be exposed by means of a scan exposure e.g. a laser or L.E.D. exposure. The latter offers the advantage that the preparation of the printing plate is simplified in that a paste-up to be used for the exposure of the imaging element can be completely prepared on a computer. This paste-up prepared on the computer is then transferred to an image setter equipped with e.g. laser that takes care of the exposure of the imaging element.
Such printing plates obtained by scan exposure are however more susceptible to wearing of the metallic silver which accepts the greasy ink. This is especially a problem for small printing details in a large non-printing area. As a consequence the printing plates are limited in printing endurance. Printing plates obtained by means of camera exposure appear to be less critical in this respect.
It was already proposed in DE 3435792 to use a matting agent having a size larger than the thickness of the base layer, in said base layer while keeping the amount of matting agent in the silver halide emulsion layer and physical development nuclei layer low. However, even if all the matting agent were included in the base layer of such printing plate, the above problem still occurs, probably because the silver halide emulsion layer may still be disturbed at places where a matting agent is present.
However by reducing the size of the matting agent below the thickness of the base layer, it was found that the printing plate tends to accept ink at the non-image areas, so-called staining.